The mineral coal is a complex mineral of widely varying composition and structure, dependent upon the location and conditions under which is was formed in nature. In general, coal is classified or ranked according to its content of volatile matter which can range from around 50% or more for lignite or cannel coal to about 20-30% for a middle rank bituminous, gas or coking to 10% or less for a high ranking bituminous coal or anthracite, the remainder being constituted by non-volatile or fixed carbon together with minor amounts of about 8% or so each of ash and moisture.
Oil shale and tar sands, similarly, vary widely in composition and structure, although both are considered to be greatly enriched in aliphatic material relative to coal. Oil shales can exist as true shale containing trapped tars and hydrocarbons, or as marls (carbonate rocks containing tars and hydrocarbons). Tar sands exist primarily as sandstones containing heavy tars and pitches. Organic content typically ranges from 4% to 60% of the total mined weight.
Pyrolytic destructive distillation has been the typical approach for fuel and resource extraction from coal, oil shale and tar sands. In all cases, volatile tars and oils are driven off, and a non-volatile solid residue (called coke if produced from coal, or petroleum coke if made from oil shale or tar sands) remain. These products are remarkably similar conidering the variability in starting material. Coke from coal has long been valuable as a fuel in the production of iron and steel and in the production of gases for heating and illumination. Petroleum coke has been used as boiler fuel. Volatile tars and oils are valuable in themselves by virtue of the inclusion therein of a large number of organic chemicals having valuable utility in industry in themselves or as intermediates for the formation of technologically important derivatives. There are now known to be contained in coal tar extracted from coal nearly 300 different organic chemical compounds including benzene and is alkylated and partially or totally hydrogenated derivatives, styrene, napthalene, and anthracene and their derivatives together with numerous other carbocyclic and heterocyclic hdyrocarbons, particularly those based on fused ring systems.
The temperature and other conditions of the pyrolytic decomposition or carbonization of coal, oil shale, or tar sands can vary considerably in order to tailor the output of the process to exaggerate the formation of certain particularly desirable compounds. Where the process conditions are selected as to be especially severe, it is usually referred to as a gasification process, the object of these conditions being to magnify the gaseous content of the reaction as greatly as possible. These vapor phase products can be condensed to produce oil fractions useful directly or by intermediate conversion, as by catalytic reforming and/or cracking as diesel oil and gasoline for internal combustion engines. Direct hydrocarbonization gasification processes subject coal to hydrogen gas under pressure in the order of about 50-100 atmospheres and are consequently expensive and difficult to practice, although such processes have become increasingly the object of concentrated research as an alternative source of internal combustion engine fuel to natural petroleum. Direct hydrogenation under somewhat milder conditions has been employed as a means to upgrade oils and tars extracted from oil shale. U.S. Pat. Nos. 3,661,423 and 3,736,246 deal with such pyrolytic extractions of oil shale. The following U.S. Pat. Nos. describe oil extraction from tar sands.
______________________________________ 3,623,971 3,844,937 3,953,317 4,242,195 3,811,506 3,925,189 4,054,506 4,238,315 3,802,508 3,913,672 4,120,775 4,250,964 3,856,474 3,951,457 4,139,450 4,248,683 3,941,679 4,284,139 ______________________________________
Coal can be subjected to so-called direct liquefaction processes in which the coal is treated under less severe conditions than utilized for carbonization and gasification, usually under pressure at temperatures below about 600.degree. C. at which substantial gas formation is initiated. Even at these conditions, coal is difficult to dissolve, and heavy attention has been directed in research in this field to the identification of solvents capable of dissolving the coal. For the most part, the solvents found to be more or less effective have been based on hydrogen-rich or protonic organic liquids, usually derived from the coal itself or as specialized by-products from the distillation and fractionation of petroleum, having a chemical structure adapted to compensate the natural hydrogen deficiency of coal which tends to impede its dissolution. Such processes are frequently carried out under high pressure in a hydrogen atmosphere to make available additional hydrogen atoms for combination with the coal. The following is a list of U.S. Pat. Nos. which relate to this kind of coal liquefaction processes:
______________________________________ 2,572,061 3,705,092 3,867,275 4,052,291 3,375,188 3,726,785 3,870,621 4,052,292 3,379,638 3,849,287 3,956,436 4,189,373 3,642,608 3,852,183 4,040,941 ______________________________________
Even though a fraction of the reaction products from the direct liquefaction process may be withdrawn and recycled for combination with fresh amounts of coal, these processes are fundamentally independent of the derivation of solvents directly from natural energy materials which might be better used for their usual purposes. In addition, versions of these processes can be carried out in the presence of finely divided solid catalysts serving to increase the efficiency of the reaction and/or bias the reaction toward the formation of particularly desirable end products such as gasoline and diesel oil. These catalysts inherently tend to become poisoned in time so as to lose their effectiveness. Separation and purification steps for the liquefaction products are seriously susceptible to clogging which requires cleaning and replacement from time to time.
In the rare instances in the art where coal has been subjected to simple extraction, e.g. U.S. Pat. No. 2,242,822, preliminary oxidation of the coal has been indispensable to convert it into a form susceptible to dissolution in furfural and furane derivatives employed as solvents. It is disclosed in U.S. Pat. No. 4,272,356 that a substituted urea or phosphoramide or sulfuramide solvent is effective to extract the non-fixed carbon content of coal at mild conditions of temperature and pressure, and without hydrogen content.